1. Field of the Invention
The present invention concerns wavelength division multiplex guided optical telecommunication systems. It is directed in particular to minimizing crosstalk between the various spectral channels occupied by the signals to be transmitted.
2. Description of the Prior Art
This requires low crosstalk optical amplifiers. Prior art amplifiers of this kind comprise optically pumped erbium doped optical fibers. They have the drawback of being difficult to integrate with semiconductor optical components.
Other optical amplifiers are in semiconductor form. One amplifier of this kind includes;
an active waveguide formed in a semiconductor substrate to guide a multiplex made up of optical signals to be amplified occupying respective signal channels in the spectrum, the gain band of the waveguide including said signal channels, PA1 two layers of two opposite conductivity types (N and P) formed in the substrate on opposite sides of the guide to enable a supply electrical current to inject charge carriers into the waveguide, and PA1 electrodes through which said supply electrical current enters a segment of the length of the waveguide constituting a gain segment. PA1 to improve the operation of a telecommunication system using low crosstalk amplifiers, PA1 to limit the cost of these amplifiers, and PA1 to facilitate the implementation of such systems by enabling simple fabrication of integrated components including such amplifiers. PA1 a waveguide formed in a semiconductor substrate to guide a multiplex comprising a plurality of optical signals to be amplified occupying respective signal channels in the spectrum, said guide means being adapted to recombine charge carriers of two opposite conductivity types to provide a gain for optical waves in a spectral band constituting a gain band including said signal channels, said waveguide having a length and a direction at each point, PA1 two layers of two opposite conductivity types (N and P) formed in said substrate on opposite sides of said waveguide to enable a supply electrical current to inject said charge carriers into said waveguide, PA1 electrodes through which an electrical supply current conferring said gain on said waveguide enters a segment of the length of said waveguide constituting a gain segment, PA1 a stabilizer system comprising at least one Bragg reflector coupled to said waveguide and having lines of equal index perpendicular to the direction of said waveguide, said system being tuned to a resonant wavelength included in said gain band but outside said signal channels and including said gain segment in a resonant cavity to sustain therein optical oscillation at said wavelength, and, PA1 at least one rejector reflector coupled to said waveguide outside said resonant cavity, said rejector reflector being a Bragg reflector tuned to said resonant wavelength and having lines of equal index inclined to said direction of said waveguide to reject out of said waveguide unwanted light emitted at said wavelength by said cavity.
Unfortunately an amplifier of this kind features high crosstalk up to modulation frequencies of several gigahertz, at least when the amplifier is operating near saturation. To a first approximation the gain is a linear function of the electrical charge carrier density in the active waveguide. This carrier density is the result of equilibrium in between the injection of carriers by a supply electrical current and the recombination of these carriers by stimulated emission related to the amplification of the input multiplex. Any modification of the overall power of the multiplex modifies the carrier density and therefore the gain of the amplifier. As this power increases the gain decreases. Conversely, as it decreases, the gain returns to its initial value.
These gain variations are very fast. After an instantaneous interruption of the input multiplex the time to recover the gain is in the order of 200 picoseconds. The gain therefore varies with the binary data modulating the amplified signals, despite the very high modulation frequency usually employed in optical telecommunications. These fluctuations in the gain generate said crosstalk between the various channels.
This crosstalk could be reduced by operating the amplifier far below saturation. This would reduce the signal to noise ratio, however, which would degrade transmission quality.
This is why, to limit crosstalk, one prior art semiconductor optical amplifier further includes a stabilizer system comprising two or at least one Bragg reflector coupled to said waveguide and having lines of equal index perpendicular to the direction of the waveguide. This system is tuned to a resonant wavelength included in said gain band but outside said signal channels. It includes the gain segment in a resonant cavity to sustain therein optical oscillation at this wavelength.
As soon as this stabilizing oscillation is present, the gain of the active material is locked at the level required to compensate the losses of the cavity, and no longer varies with the incoming power: the amplitude variations of the input multiplex are compensated by the amplitude variations of the stabilizing oscillation.
One such prior art low crosstalk amplifier is known as a "stabilized gain" amplifier. It is described in French patent application 93 10147 filed 20 Aug. 1993 (publication No. 2 709 189).
Despite the advantages it has over prior art amplifiers, it has become clear that the operation of telecommunication systems using this low crosstalk amplifier is less than optimal in some cases.
The present invention has the following aims: